This invention relates to an ultrasonic imaging device and methods for use and manufacture thereof, and particularly to an ultrasonic imaging device positionable in coronary vessels to obtain images thereof.
Ultrasonic imaging of portions of a patient's body provides a useful tool in various areas of medical practice for determining the best type and course of treatment. Imaging of the coronary vessels of a patient by ultrasonic techniques could provide physicians with valuable information about the extent of a stenosis in the patient and help in determining whether procedures such as angioplasty or atherectomy are indicated or whether more invasive procedures may be warranted. However, obtaining ultrasonic images of the distal coronary vessels with sufficiently high resolution to be valuable for making medical decisions, such as described above, requires overcoming several significant obstacles one of the most significant of which relates to the size of the ultrasonic sensing device.
Obtaining ultrasonic images of high resolution of a body organ generally requires bringing an ultrasonic sensor (i.e. a transmitter/receiver) sufficiently proximate to the organ and scanning the organ with ultrasonic pulses. Ultrasonic imaging of organs deep within the body that are surrounded by other, relatively dense organs and tissues requires connecting a sensor on a probe and positioning the sensor and the probe near or even into the organ. The heart and the vessels connected to it are organs of this type. Because it is a well known technique to insert catheters, guide wires and probes into the coronary vasculature from remote sites via arteries, such as the femoral artery, and further because some of the information of interest to the physician is the extent of stenosis on the inside walls of the coronary vessels, it would be desirable to be able to position an ultrasonic sensor connected to a probe into the distal regions of the coronary vasculature via a remote arterial site, such as the femoral artery, to obtain ultrasonic images of the coronary arterial walls.
The vessels in the distal regions of the vascular tract that would be useful to image include the coronary arteries, branch vessels stemming from the external carotid artery such as the occipital and the arteries leading to the vessels of the head and brain, splenic, and the inferior mesenteric and renal arteries leading to the organs of the thorax. To be positioned in these regions, the size of an ultrasonic sensor and probe must be relatively small not just to traverse the arterial vessel but also to avoid occluding the vessel lumen. When a device, such as a catheter, probe, or sensor, is positioned in a blood vessel, it occupies a volume which restricts blood flow within the vessel as well as in vessels proximate thereto. When a device is positioned within an arterial vessel, the blood flow through the vessel is restricted to an annular region (i.e. the area of "ring"-shaped cross section) which is effectively created between the outer perimeter of the device and the inner wall of the vessel. This would normally not present a problem in large arteries with large blood flows, such as the femoral arteries of the legs, or the aorta, or in very proximal coronary arteries. In these large arteries, any restriction caused by the device would be relatively small and the blood flow would be relatively large. However, in small arteries in remote locations, such as the occipital that leads to the brain, or the coronary arteries of sizes of 3.0 mm or less that lead to the right and left sides of the heart, any restriction of blood flow must be minimized. The consequences of occluding these small vessels can cause a loss of flow in the coronary arteries of the heart which may have several adverse effects, such as severe chest pains, or physiological changes such as arrythmia, ischemia, and tachycardiac response. These effects may be threatening to the patient and further, once begun, may be difficult to stabilize.
Moreover, not only are these latter vessels very small but these vessels are also those in which there might also be restrictive disorders, such as atherosclerosis. Atherosclerotic disease as well as other thrombus formations which occlude blood flow occurs in these smaller arteries due to the hemodynamics of the blood tissue interface. Reflecting this fact is that presently angioplasty is primarily performed in vessels of a size range of 2.0 to 3.5 mm in diameter. Such disorders would diminish the cross sectional area of these vessel lumens even more.
Therefore, a significant obstacle to using an intravascular probe device to obtain ultrasonic images of such vessels is that the probe should be sufficiently small in dimension so as not only to be positioned in these small, possibly partially occluded arteries, but also to be sufficiently small so as not to totally or almost totally occlude the lumen of the vessel into which it is positioned. Accordingly, for an ultrasonic sensor device to be used for distal coronary applications, it must be small enough to be suitably positioned in the coronary vessels and to permit a sufficient blood flow therearound. For use in the distal coronary vasculature, the exterior dimension for a sensor device should be approximately 0.040 inches (1 mm) in diameter to provide an annular region of flow in even the most distal vessels.
Ultrasonic imaging devices intended to be placed in the vascular system have been disclosed in prior patents (e.g. U.S. Pat. No. 4,794,931). However, these prior devices have had numerous drawbacks that limited their utility for the most part to uses in only the peripheral vasculature and not in deep coronary arteries. Prior devices, having diameters ranging from 3.5 French (1.2 mm) and up, would be limited by their size to only very proximal coronary arteries. Prior devices, having diameters ranging from 4.5 French (1.5 mm) and up, would be limited by their size to only very proximal locations of coronary arteries, peripheral vessels, or very proximal organ vessels. Furthermore, in addition to these limitations, prior ultrasonic probe devices have produced images lacking in sufficiently high detail and resolution to provide useful information.
There are significant obstacles to making an ultrasonic probe device with dimensions sufficiently small to fit into distal coronary vessels and yet possessing the necessary mechanical and electrical properties required for high quality ultrasonic images. For example, in order to position a probe device in a deep coronary vessel from a remote percutaneous site such as via the femoral artery, the probe device should possess a high degree of longitudinal flexibility over its length. The vessel paths of access to such deep coronary vessels, as well as the numerous branches which stem from these vessels, may be of an extremely tortuous nature. In some areas within the vascular system, an ultrasonic probe device may have to transverse several curvatures of radius of 3/16 of an inch (4.7 mm) or less. Thus, the probe device should possess a high degree of flexibility longitudinally over its length to enable it to transverse virtually any curvature of the vascular tract.
Another desired mechanical property for the probe device is stable torsional transmittance. If the device is to include a rotating ultrasonic sensor at a distal end to make radial scans of the entire cross section of the coronary artery, it should not only be flexible longitudinally, but should also be stiff torsionally. Rotation of the ultrasonic device should be achieved so that a drive shaft extending to the sensor does not experience angular deflection which might cause image distortion. Due to the continuous angular motion which dictates the location at which an ultrasound sensor scans, if angular deflection occurs at the distal end of sensor drive shaft, it can result in an artifact of angular distortion that becomes apparent on the ultrasound displayed image. This artifact can occur if the rotating sensor drive shaft experiences "whip". "Whip" may be defined as the angular deceleration and acceleration of the sensor drive shaft as a result in shaft angular deflections during rotation. As the transducer drive shaft is rotating it may undergo angular deflection if the drive shaft's torsional stiffness is low enough to make the drive shaft susceptible to dynamic changes in torsional load. It may also undergo angular deflection if the dynamic torsional loads are high and varying.
During operation, relative changes in torsional load should be minimal, therefore any induced `whip` could be attributed to a shaft with a low torsional stiffness. The acceleration and decelerations associated with shaft whip can be described by the energy change from kinetic to potential under varying torsional load conditions. For example, as a sensor drive shaft encounters additional torsional resistance its angular velocity drops causing a deceleration and shaft angular deflection. When the shaft is free of the added resistance, the energy stored in the shaft, in the form of potential energy from the angular deflection and shaft stiffness, is released causing an angular acceleration and increase in the shaft's angular velocity.
Image quality and accuracy is dependent on constant sensor angular velocity. Image construction assumes a constant sensor velocity, therefore relative acceleration or deceleration between the expected and actual sensor angular velocity will cause image distortion.
Even if a sensor possesses the aforementioned minimal size and mechanical properties, the value of the device depends upon the quality of the ultrasonic image which in turn is a direct function of both the acoustic pulse signal and the electrical signal transmission. Therefore, in addition to the mechanical properties necessary for locating and rotating a sensor, the device should also provide a high quality electrical and acoustic signal. This may include several specific properties, such as a high signal to noise ratio of the electronic signal, impedance matching of the overall system without the need for internal electronic matching components, and minimization of voltage requirements to attain a signal of sufficient quality to provide an image.
Accordingly, it is an object of the present invention to provide a device that overcomes the limitations of the prior art and which enables the ultrasonic scanning of small diameter body vessels with a transducer probe that can be positioned therein.
It is a further object of the invention to provide an apparatus, and methods for use and manufacture, that enables obtaining ultrasonic image information of high resolution or quality.